1,076 research outputs found

### Nonequilibrium multicritical behavior in anisotropic Heisenberg ferromagnet driven by oscillating magnetic field

The Heisenberg ferromagnet (uniaxially anisotropic along z-direction), in the
presence of time dependent (but uniform over space) magnetic field, is studied
by Monte Carlo simulation. The time dependent magnetic field was taken as
elliptically polarised in such a way that the resulting field vector rotates in
the XZ plane. In the limit of low anisotropy, the dynamical responses of the
system are studied as functions of temperature and the amplitudes of the
magnetic field. As the temperature decreases, it aws found that the system
undergoes multiple dynamical phase transitions. In this limit, the multiple
transitions were studied in details and the phase diagram for this observed
multicritical behaviour was drawn in the field amplitude and temperature
palne.The natures (continuous/discontinuous) of the transitions are determined
by the temperature variations of fourth order Binder cumulant ratio and the
distributions of the order parameter near the transition points. The
transitions are supported by finite size study. The temperature variations of
the variances of dynamic order parameter components (for different system
sizes) indicate the existence of diverging length scale near the dynamic
transition points. The frequency dependences of the transition temperatures of
the multiple dynamic transition are also studied briefly.Comment: 14 Pages Latex, 17 Postscript figures. To appear in Int. J. Mod.
Phys. C (2006) Ma

### Simulations of Information Transport in Spin Chains

Transport of quantum information in linear spin chains has been the subject
of much theoretical work. Experimental studies by nuclear spin systems in
solid-state by NMR (a natural implementation of such models) is complicated
since the dipolar Hamiltonian is not solely comprised of nearest-neighbor
XY-Heisenberg couplings. We present here a similarity transformation between
the XY-Heisenberg Hamiltonian and the grade raising Hamiltonian, an interaction
which is achievable with the collective control provided by radio-frequency
pulses in NMR. Not only does this second Hamiltonian allows us to simulate the
information transport in a spin chain, but it also provides a means to observe
its signature experimentally

### From Disordered Crystal to Glass: Exact Theory

We calculate thermodynamic properties of a disordered model insulator,
starting from the ideal simple-cubic lattice ($g = 0$) and increasing the
disorder parameter $g$ to $\gg 1/2$. As in earlier Einstein- and Debye-
approximations, there is a phase transition at $g_{c} = 1/2$. For $g<g_{c}$ the
low-T heat-capacity $C \sim T^{3}$ whereas for $g>g_{c}$, $C \sim T$. The van
Hove singularities disappear at {\em any finite $g$}. For $g>1/2$ we discover
novel {\em fixed points} in the self-energy and spectral density of this model
glass.Comment: Submitted to Phys. Rev. Lett., 8 pages, 4 figure

### The Trapped Polarized Fermi Gas at Unitarity

We consider population-imbalanced two-component Fermi gases under external
harmonic confinement interacting through short-range two-body potentials with
diverging s-wave scattering length. Using the fixed-node diffusion Monte Carlo
method, the energies of the "normal state" are determined as functions of the
population-imbalance and the number of particles. The energies of the trapped
system follow, to a good approximation, a universal curve even for fairly small
systems. A simple parameterization of the universal curve is presented and
related to the equation of state of the bulk system.Comment: 4 pages, 2 tables, 2 figure

### Electric Control of Spin Currents and Spin-Wave Logic

Spin waves in insulating magnets are ideal carriers for spin currents with
low energy dissipation. An electric field can modify the dispersion of spin
waves, by directly affecting, via spin-orbit coupling, the electrons that
mediate the interaction between magnetic ions. Our microscopic calculations
based on the super-exchange model indicate that this effect of the electric
field is sufficiently large to be used to effectively control spin currents. We
apply these findings to the design of a spin-wave interferometric device, which
acts as a logic inverter and can be used as a building block for
room-temperature, low-dissipation logic circuits.Comment: 4 pages, 3 figures, added the LL equation and the discussion on
spin-wave-induced electric field, accepted by PR

### Theory of optical spectral weights in Mott insulators with orbital degrees of freedom

Introducing partial sum rules for the optical multiplet transitions, we
outline a unified approach to magnetic and optical properties of strongly
correlated transition metal oxides. On the example of LaVO$_3$ we demonstrate
how the temperature and polarization dependences of different components of the
optical multiplet are determined by the underlying spin and orbital
correlations dictated by the low-energy superexchange Hamiltonian. Thereby the
optical data provides deep insight into the complex spin-orbital physics and
the role played by orbital fluctuations.Comment: 6 pages, 3 figures, expanded versio

### Anomalous dynamics in two- and three- dimensional Heisenberg-Mattis spin glasses

We investigate the spectral and localization properties of unmagnetized
Heisenberg-Mattis spin glasses, in space dimensionalities $d=2$ and 3, at T=0.
We use numerical transfer-matrix methods combined with finite-size scaling to
calculate Lyapunov exponents, and eigenvalue-counting theorems, coupled with
Gaussian elimination algorithms, to evaluate densities of states. In $d=2$ we
find that all states are localized, with the localization length diverging as
$\omega^{-1}$, as energy $\omega \to 0$. Logarithmic corrections to density of
states behave in accordance with theoretical predictions. In $d=3$ the
density-of-states dependence on energy is the same as for spin waves in pure
antiferromagnets, again in agreement with theoretical predictions, though the
corresponding amplitudes differ.Comment: RevTeX4, 9 pages, 9 .eps figure

### A modified triplet-wave expansion method applied to the alternating Heisenberg chain

An alternative triplet-wave expansion formalism for dimerized spin systems is
presented, a modification of the 'bond operator' formalism of Sachdev and
Bhatt. Projection operators are used to confine the system to the physical
subspace, rather than constraint equations. The method is illustrated for the
case of the alternating Heisenberg chain, and comparisons are made with the
results of dimer series expansions and exact diagonalization. Some discussion
is included of the phenomenon of 'quasiparticle breakdown', as it applies to
the two-triplon bound states in this model.Comment: 16 pages, 12 figure

### Magnetic structure and phase diagram in a spin-chain system: Ca$_3$Co$_2$O$_6$

The low-temperature structure of the frustrated spin-chain compound
Ca$_3$Co$_2$O$_6$ is determined by the ground state of the 2D Ising model on
the triangular lattice. At high-temperatures it transforms to the honeycomb
magnetic structure. It is shown that the crossover between the two magnetic
structures at 12 K arises from the entropy accumulated in the disordered
chains. This interpretation is in an agreement with the experimental data.
General rules for for the phase diagram of frustrated Ising chain compounds are
formulated.Comment: 4 pages, 2 figure

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